1. Field of the Invention
The present invention generally relates to thin film light emitting devices and, more particularly, is directed to a thin film light emitting device for use with a thin film color display apparatus or the like.
2. Description of the Related Art
As a thin film light emitting device, the development of an electroluminescence (EL) devices has been advanced so far. The conventional EL device has electrodes formed on both surfaces of a phosphor thin film and a voltage is applied to these electrodes to thereby make the phosphor become luminous.
A principle of so-called electric field radiation is considered as follows:
An electric field whose magnitude is large in the thickness direction of the phosphor thin film, e.g., electric field of about 10.sup.6 V/cm is generated in the phosphor thin film by the voltage applied to these electrodes. By this electric field thus generated, electrons of surface level of phosphoror of impurity level are emitted by a so-called tunnel effect to the conduction band. Further, hot electrons are generated by an acceleration of the electric field and the hot electrons strike radiation centers in the phosphor, whereby the radiation centers are set in an excited state by the reception of energy. Then, when the radiation centers return to a ground state, photons are emitted.
In such electric field radiation, when ZnS is employed as a host crystal of phosphor, efficient radiation is obtained in the radiation center of internal transition type such as the radiation center of Mn or rare-earth materials.
However, bright radiation is not obtained in the radiation center of donor acceptor pair type such as ZnS : Cu, Al or ZnS : Ag, Al which demonstrate high radiation efficiency by the excitation of electron beam.
Accordingly, in this kind of electric field radition type thin film light emitting device, light emitting elements of various colors, particularly a blue light emitting element cannot be obtained without difficulty in the prior art, which becomes a bottleneck in the application of this kind of thin film light emitting device to a thin film color video display apparatus.